September 24th, 2006
The Casson family series (Saffy's Angel, Indigo's Star, Permanent Rose, Caddy Ever After), by Hilary McKay--Sarah (though she may yet have her own book; I'm not the only one who'd be happy if she did)
Then There Were Five, by Elizabeth Enright--Mark Herron
Nightbirds on Nantucket, by Joan Aiken-- I suppose Dido Twite doesn't count, as she gets several of her own books later, but she's a very sassy sidekick in this one!
--elswhere, here via Big A Little A